From what I've noticed this is one of the rare examples in whichthe verb ir is used as the auxiliary verb in the passive voice.

With other verbs, this is not the case, you never see things likeFulana vai morta, Fulano vai atropelado...

The only explanation I can think of, with the verb prender (in the sense of to jail, to lock up),people use the expression IR PRESO, which is not a passive voice at all, but an idiomatic expression meaning ''to go to jail''. ''ir preso por + X'' (to go to jail because of).

There are people in this forum better able to explain that than me. But I can say I always read "vai preso" as the actual travel of the person to the jail, inside a car. Like "goes locked to the jail".